//This is a <div> that wraps around your listboxes
var wrapperDiv = document.getElementById('listboxWrapper');

//Get all <select> elements within the <div>
var sels = wrapperDiv.getElementsByTagName('SELECT');

//An array to store the width values
var widths = new Array();

//Load the array
for(var i = 0, l = sels.length; i < l; i++)
  widths[i] = sels[i].offsetWidth;

//Get the max width
var maxW = Math.max(widths);

//Set the max width to all the list boxes
for(var sel in sels)
  sels[sel].style.width = maxW;

var width = 0;
//get the largest width
$("select").each(function() {
    if ($(this).width() > width) {
        width = $(this).width();
     }
});
//make them all the same width
$("select").css("width", width);